Alkaline strip passivation

ABSTRACT

Process for the alkaline passivation of galvanized and alloy-galvanized steel surfaces and of aluminum in strip lines, wherein the metal surfaces are brought into contact with an alkaline aqueous solution that contains complexing agents and iron, which has a free alkalinity in the range of 5 to 20 points and a total alkalinity in the range of 7 to 30 points and contains:0.1 to 1 g/l magnesium ions,0.1 to 1 g/l iron (III) ions and0.5 to 5 g/l amino or hydroxy carboxylate ions. This may be followed by a chrome-containing or chrome-free secondary passivation.

This application is filed under 35 U.S.C. 371 and based onPCT/EP98/04670, filed Jul. 25, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the alkaline passivation ofgalvanized and alloy-galvanized steel surfaces and of aluminum andalloys thereof in strip lines. As a result of this passivation, ananticorrosive coat is formed which may serve as a base for subsequentlacquering.

2. Discussion of Related Art

Alkaline passivation processes have long been known for treating themetal surfaces mentioned. In order to achieve adequate corrosionprotection and the required lacquer adhesion, the treatment solutionsgenerally contain toxic heavy metals, such as nickel ions and/or cobaltions. These ions are held in solution under the alkaline conditions bysuitable complexing agents. Examples of such passivation solutions aregiven in EP-A-405 340.

Satisfactory results in terms of corrosion protection and lacqueradhesion are obtained as a result of the presence of nickel and/orcobalt ions in the alkaline passivation solution. However, owing to thehigh toxicity, the necessary workers' protection measures and the costlywaste disposal, there are reservations about the use of toxic heavymetals, such as nickel or cobalt. An objection of the present inventionis therefore providing a process for the alkaline passivation ofsuitable metal surfaces, the efficiency of which is similar to that ofnickel or cobalt-containing processes, but which does not use toxicheavy metals.

This object is achieved by a process for the alkaline passivation ofgalvanized and alloy-galvanized steel surfaces and of aluminum in striplines, wherein the metal surfaces are brought into contact with analkaline aqueous solution which contains complexing agents and iron,wherein the aqueous solution has a free alkalinity in the range of 5 to20 points and a total alkalinity in the range of 7 to 30 points andcontains:

0.1 to 1 g/l magnesium ions,

0.1 to 1 g/l iron (III) ions and

0.5 to 5 g/l amino or hydroxycarboxylate ions.

“Aluminum” here means aluminum and aluminum alloys which contain morethan 50% w/w aluminum.

“Alkaline” means that the treatment solutions have a pH of more than9.5, preferably in the range of 10 to 13. In practice, the freealkalinity and total alkalinity contents are used rather than the pHvalues to characterize the treatment baths. The free alkalinity isdetermined by titrating 2 ml of bath solution, preferably diluted to 50ml, with a 0.1 N acid, such as hydrochloric acid or sulfuric acid, to apH of 8.5. The acid solution consumed in ml gives the point value of thefree alkalinity. The total alkalinity is determined in the same way bytitrating 2 ml of treatment solution, diluted to 50 ml, with 0.1 N acidto a pH of 4.0. This necessarily means that the total alkalinity ishigher than the free alkalinity.

The treatment solution may contain additional active substances whichhave a positive effect on coat formation and corrosion protection.Contents of about 0.001 to about 20 g/l of fine-particle SiO₂, forexample pyrogenic silica, may be mentioned as an example. Pyrogenicsilica having a specific surface (BET) in the range of about 150 toabout 300 m²/g, for example about 200 m²/g, is preferably used. Aqueoussuspensions of such silicas are commercially available.

A further optional component of the treatment solution is about 0.001 toabout 10 g/l of monomeric or oligomeric acrylate and/or methacrylateions. These may be added to the treatment solution as soluble salts, forexample sodium salts, or in the form of the free acid. In the subsequentdrying step, at the latest when a lacquer that has been applied isstoved, these ions polymerise, thus improving the coat properties.

The amino or hydroxycarboxylate ions serve to prevent the precipitationof iron and magnesium compounds at the alkaline pH of the treatmentsolution. In principle, all amino or hydroxycarboxylate ions which bringabout this effect are suitable for this purpose. They may be introducedinto the treatment solution in the form of soluble salts, such as sodiumsalts, or as free acids. Monobasic hydroxycarboxylic acids with at leastfour carbon atoms and at least two alcoholic hydroxyl groups arepreferred. Gluconate ions or the one carbon atom longer glucoheptanateions are particularly preferred.

Apart from these constituents, the treatment solutions also contain theanions of the salts in the form of which the magnesium and iron (III)ions were introduced into the treatment solution. The nitrates arepreferably used since nitrate ions do not have a negative effect oncorrosion protection.

The process is designed for the treatment of continuous metal strips, sothe treatment time is in the range of just a few seconds, depending onthe line speed and length of the treatment zone. The duration oftreatment may, for example, be about 3 to about 30 seconds and ispreferably in the range of 5 to 15 seconds. The metal surfaces arepreferably brought into contact with an aqueous treatment solution whichhas a temperature in the range of about 40 to about 70° C. Spraying,dipping and roll coating processes are suitable methods of contacting.Spray application is usual and is suitable in the present context. Atthe end of the desired reaction time, the aqueous treatment solution isrinsed off.

The corrosion protection effect may be improved further if thealkalinically-passivated metal surfaces are subjected to secondarypassivation. Chrome-containing processes are widespread in the art forsecondary passivation, following alkaline passivation. Chrome-containingsecondary passivation is also suitable as an after-treatment followingthe alkaline passivation according to the present invention. However, aparticular advantage of the process according to the present inventionlies in the fact that an ecologically and toxicologically problematicchrome-containing after-treatment may be omitted and instead, achrome-free secondary passivation may be carried out. Aqueous solutionswhich contain titanium and/or zirconium compounds, particularly complexfluorides of these elements, are particularly suitable as chrome-freesecondary passivation solutions. For example, such a secondarypassivation solution may contain a total of 0.4 to 10 g/l of complexfluorides of titanium and/or zirconium. The pH of this chrome-freesecondary passivation solution is preferably in the range of about 2 toabout 4.

The alkaline passivation step according to the present invention thusrepresents one treatment step in a multi-step chain of treatment inwhich the metal strips are usually first degreased and rinsed. Thealkaline passivation according to the present invention follows this,usually followed by the steps of rinsing, secondary passivation, usuallyrinsing again, drying (at about 60 to about 100° C. object temperature),lacquering.

In principle, the alkaline passivation solution may be prepared in situby dissolving the individual constituents in water in the concentrationrequired for use. In practice, however, it is usual to make concentratesand to transport them to the treatment plant where the ready-to-usetreatment solution is prepared by simply diluting with water.Accordingly, the present invention also comprises an aqueous concentratewhich, when diluted with water by a factor in the range of 7 to 15,gives an aqueous treatment solution for the alkaline passivation ofgalvanized and alloy-galvanized steel surfaces and of aluminum in striplines, which has a free alkalinity in the range of 5 to 20 points and atotal alkalinity in the range of 7 to 30 points and contains:

0.1 to 1 g/l magnesium ions,

0.1 to 1 g/l iron (III) ions and

0.5 to 5 g/l amino or hydroxycarboxylate ions.

In producing these concentrates, preferably magnesium and iron salts,preferably nitrates, are first dissolved in water to the requiredconcentration, the complexing carboxylic acid is then added and onlythen is the alkaline pH adjusted by adding lye, for example sodiumhydroxide solution.

DESCRIPTION OF THE INVENTION EXAMPLES

The following Examples show that the alkaline passivation processaccording to the present invention leads to corrosion protection resultssimilar to those of comparable known processes which contain toxicnickel and/or cobalt ions. The galvanized and alloy-galvanized steelplates mentioned in Table 2 were alkalinically-passivated according toTable 1, rinsed with water for 5 seconds, subjected to secondarypassivation using chrome-containing or chrome-free secondary rinsingsolutions (treatment time: 5 seconds spraying), rinsed using deionisedwater for 2 seconds and dried in a circulating-air drying cupboard at acupboard temperature of 70° C. For the lacquering, 5 μm of primer and 18μm of top coat, both polyester-based, were applied and stoved at 216 or249° C. The test plates were then subjected to a 42-day salt spray testaccording to German standard DIN 50 021 SS. The results (lacquercreepage in mm on the section and on the edge) are reproduced in Table2. In addition, lacquer adhesion was investigated by a T-bend test. Nosignificant differences were shown between the process according to thepresent invention and the comparable processes using nickel and/orcobalt.

TABLE 1 Alkaline passivation solutions and treatment conditions FA¹⁾TA²⁾ Temp Time Co Ni Fe Mg Complex³⁾ SiO₂ ⁴⁾ Acrylic Secondary Ex. No.(ml) (ml) (° C.) (s) g/l g/l g/l g/l g/l g/l g/l rinse used Comp. 1 5 755 10 0.29 0 0.21 0 1.8 0 0 Chrome⁵⁾ Comp. 2 5 7 55 10 0.29 0 0.21 0 1.80 0 Chrome-free⁶⁾ Comp. 3 6 8 60 20 0.29 0.21 0.21 0 1.8 0 0 ChromeComp. 4 6 8 60 20 0.29 0.21 0.21 0 1.8 0 0 Chrome-free Comp. 5 5 7 60 300.16 0 0.21 0.23 1.8 8.6 0 Chrome Ex. 1 7 9 60 15 0 0 0.21 0.33 1.8 0 0Chrome Ex. 2 6 8 60 15 0 0 0.21 0.33 1.8 0 0 Chrome-free Ex. 3 5 7 60 100 0 0.21 0.33 1.8 8.6 0 Chrome Ex. 4 5 7 60 10 0 0 0.21 0.33 1.8 8.6 0Chrome-free Ex. 5 6 8 60 10 0 0 0.21 0.33 1.8 0 5.7 Chrome-free Ex. 6 57 65 10 0 0 0.15 0.15 0.8 0.5 0 Chrome-free Ex. 7 19  29  45 10 0 0 9.901.0 5.0 0 9.2 Chrome-free Ex. 8 10  15  60 10 0 0 0.21 0.6 2.4 17 0Chrome-free ¹⁾Free alkalinity (cf. text) ²⁾Total alkalinity (cf. text)³⁾Complexing agent: glucoheptanate ⁴⁾Pyrogenic silica; specific surface200 m²/g ⁵⁾Chrome-containing secondary passivation solution (1.5 g/l Cr(VI); pH 3.2) ⁶⁾Chrome-free secondary passivation solution based on TiF₆²⁻ (0.5 g/l, pH 3.4)

TABLE 2 Corrosion test results (42-day salt spray test) AZ³⁾ Z¹⁾ ZA²⁾Creepage in Creepage in mm on Creepage in mm on mm on Treatment sectionedge section edge section edge Comp. 1 2.5 24 3 17 4.5 30 Comp. 2 2.7 186.5 18 4.5 30 Comp. 3 1.8 24 0.2 8 0.4 13 Comp. 4. 4.5 18 0.2 7 4.1 21Comp. 5 1.6 30 0.6 7 0.4 40 Example 1 2.8 27 0.7 7 0.4 22 Example 2 3.217 5 16 1.7 8 Example 3 1.5 32 1.3 8 0.5 34 Example 4 1.2 26 0.4 8 1.126 Example 5 2.2 10 3 16 0.8 12 Example 6 1.8 15 Example 7 1.5 13Example 8 2.4 12 ¹⁾Hot-dip galvanized steel ²⁾Alloy-galvanized steelwith 95% Zn, 5% Al (Galfan^(R)) ³⁾Alloy-galvanized steel with 45% Zn,55% Al (Galvalume^(R))

What is claimed is:
 1. A process for the alkaline passivation of a metalsurface selected from the group consistng of galvanizd steel,alloy-galvanized steel and aluminum comprising: a) forming an alkalineaqueous solution comprising: 1) 0.1 to 1 gl of magnesium ions; 2) 0.1 to1 g/l of iron (III) ions; and 3) 0.5 to 5 g/l amino orhydroxycarboxylate ions; 4) 0.001 to 10 g/l of a member selected fromthe group consisting of acrylic acid ions, methacrylic acid ions,oligomers comprising acrylic acid residues, oligomers comprisingmethacrylic acid residues and mixtures thereof; wherein said aqueoussolution has a free alkalinity in the range of 5 to 20 points and atotal alkalinity in the range of 7 to 30 points; and b) containing themetal surface with the alkaline aqueous solution.
 2. The process ofclaim 1 wherein said aqueous solution further comprises 0.001 to 20 g/lof pyrogenic silica.
 3. The process of claim 1 wherein saidhydroxycarboxylate ions are selected from the group consisting ofmonobasic hydroxycarboxylic acids with at least four carbon atoms and atleast two alcoholic hydroxyl groups.
 4. The process of claim 1 whereinsaid hydroxycarboxylate ions are selected from the group consisting ofgluconate ions, glucoheptanate ions, and mixtures thereof.
 5. Theprocess of claim 1 wherein magnesium and iron nitrate are the source ofsaid magnesium and iron (III) ions.
 6. The process of claim 1 whereinsaid aqueous solution contacts said metal surface for 3 to 30 seconds.7. The process of claim 6 wherein said aqueous solution contacts saidmetal surface for 5 to 15 seconds.
 8. The process of claim 1 whereinsaid aqueous solution contacting said metal surface is at a temperatureof from 40 to 70° C.
 9. The process of claim 1 further comprisingrinsing said metal surface with water following contact with the aqueoussolution, then treating said metal surface with a chrome-free secondarypassivation solution comprising 0.4 to 10 g/l of complex fluorides oftitanium, zirconium, or a mixture thereof.
 10. The process of claim 9wherein said secondary passivation solution has a pH of from 2 to about4.
 11. The process of claim 9 further comprising drying said metalsurface, following primary and secondary passivation treatments, at from60 to 100° C., followed by lacquering.
 12. The process of claim 1wherein an aqueous concentrate is diluted with water by a factor of from7 to 15 to provide the alkaline aqueous solution.